This invention relates to chip thennistors. More particularly, this invention relates to composite electronic devices with a resistor and a chip thermistor.
Surface-mountable chip thermistors are coming to be widely used in recent years. As is well known, chip thermistors include both the PTC type and the NTC type, and the B-constant (the resistance-temperature characteristic) of an NTC thermistor is determined by the composition of the thermistor ceramic material to be used and has been difficult to control freely. For this reason, it has been a common practice to connect a resistor in series or in parallel with a thermistor to adjust the B-constant for each circuit to be in used. This not only adversely affects the workability but also requires a larger area to individually mount a resistor and a thermistor to a circuit board as individual electronic components.
In view of the above, Japanese Patent Publication Tokkai 64-1206 has disclosed a chip thermistor having a resistor layer formed between outer electrodes on its outer surface such that the thermistor and the resistor layer are connected in parallel. This has the advantage in that the area for the surface mounting can be reduced because the thermistor and the resistor are on a single chip and also in that the B-constant of the thermistor can be freely adjusted by varying the resistance of the resistor layer.
Chip thermistors thus structured, however, have a lower reliability because the resistor layer is externally exposed. In addition, errors are likely to be committed in their mounting, that is, they are likely to be mounted erroneously with the resistor layer on the side of the circuit board.
It is therefore an object of this invention to provide compact and reliable chip thermistors of which the B-constant can be adjusted easily and errors in mounting can be obviated.
A chip thermistor embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a main body of a thermistor ceramic material having a specified resistance-temperature characteristic, outer electrodes formed on its outer end surfaces, at least one high-resistance conductor (or a xe2x80x9cresistorxe2x80x9d) and inner electrodes inside the thermistor ceramic body and wherein the resistor and at least one mutually separated pair of inner electrodes with the thermistor ceramic material in between are electrically connected either in series or in parallel. Since a thermistor and a resistor are made into one chip according to this invention, it is possible to obtain a chip thermistor which is compact and of which the B-constant can be freely adjusted. Since the resistors are not externally exposed but are formed inside the thermistor ceramic, there is no danger of their erroneously contacting an external circuit at the time of mounting the chip thermistor. In other words, it is only the outer electrodes that are externally exposed, and this improves reliability. For the purpose of the present invention, the expression xe2x80x9chigh-resistance conductorxe2x80x9d or xe2x80x9cresistorxe2x80x9d in defined as an electronic element with a much higher resistance than the inner electrodes, or an element with resistance greater than 1xcexa9, the resistance of the inner electrode being typically in the milliohm range.
The resistance value of the high-resistance conductors can be freely changed by connecting in series and/or parallel the inner electrodes facing each other and sandwiching the thermistor ceramic material in between. In order to obtain a larger resistance value, the resistors may be formed in the shape of a coil. This method is preferable because it is possible to increase the resistance value without being affected by the thermistor characteristic between the conductors.
Thermistors with negative thermistor-resistance characteristics (NTC thermistors) are widely in use for temperature compensation for a circuit element and temperature detection. The B-constant of such an NTC thermistor is determined by the material composition of the thermistor ceramics. The B-constant represents the magnitude of change in no-load resistance value against temperature and may be obtained from two arbitrary temperatures T and T0 as follows:
B={log{R/R0)}/{(1/T)xe2x88x92(1/T0)}xe2x80x83xe2x80x83Formula (1)
where T and T0 are in units of absolute temperature (K) and R and R0 are no-load temperature values at these temperatures in xcexa9. Since the ratio R/R0 changes, the B-constant can be changed although the thermistor ceramics are the same.